Wastewater is water that has been used. The wastewater usually contains various pollutants, depending on what it was used for, that can result in environmental pollution. In order to protect the environment and promote public health, wastewater treatment is required. Wastewater treatment methods are broadly classifiable into physical, chemical and biological processes.
There are many biological processes for treatment of wastewaters, for example activated sludge, trickling filters, rotating biological contactors and bio-filters. Many are aerobic wastewater treatment processes and have in common the use of oxygen (or air) and microbial action to biotreat the pollutants in wastewaters. Equipment may include diffused aeration systems or surface-aerated systems in order to maximize oxygen transfer and minimize odors as the wastewater is treated. Aeration is one of the first stages in the process since bacteria and other organisms require oxygen to aerobically break down organic substances in the wastewater being treated. In a surface-aerated system or floating surface aerator, the aerator transfers the air or oxygen required by the biological oxidation reactions into the wastewater liquid, and provides the mixing required for dispersing the air or oxygen and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kWh. However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated wastewater liquid do not achieve the same performance level as activated sludge units. An activated sludge process refers to a multi-chamber reactor unit that makes use of highly concentrated microorganisms to degrade organics and remove nutrients from wastewater to produce a high-quality effluent. To maintain aerobic conditions and to keep the activated sludge suspended, a continuous and well-timed supply of oxygen is required. Activated sludge consists of flocs of bacteria, which are suspended and mixed with wastewater in an aerated tank. The bacteria use the organic pollutants to grow and transform it to energy, water, CO2 and new cell material. Activated sludge systems are suspended-growth type and are used in conventional high-tech wastewater treatment plants to treat almost every wastewater influent as long as it is biodegradable.
For instance, a wastewater treatment system is disclosed in U.S. patent publication No. 2015/0101993. The device is placed in an aeration basin containing a surfactant laden wastewater, in which a helical impeller connected to a motor rotates within a submerged draft tube. Oxygen gas is introduced into the draft tube through a gas inlet which discharges the oxygen gas via a sintered metal disk, ceramic membrane or polymeric ultrafine pore diffuser ring to deliver very fine bubbles of high purity oxygen directly into the surfactant laden wastewater. During operation of the device, a recirculation loop below the surface of the surfactant laden wastewater is created for undissolved gas bubbles that are discharged from the exit of the draft tube and recaptured into the inlet opening of the draft tube. In order to achieve a reliable capture and recovery of undissolved gases without a collection hood or containment surface, a higher relative value of superficial liquid velocity in the draft tube to the terminal bubble rise velocity is maintained, which is controlled by the rotation speed of the helical impeller. In this manner, small bubble sizes of injected oxygen are controlled to obtain a uniform, terminal ascent velocity less than the superficial velocity of the liquid and allow a substantial portion of the gas to be recaptured for recirculation back into the draft tube. Thus, the operation of this device relies on the control of the ascent velocity of the oxygen bubbles and the superficial velocity of the liquid. This requires attention on the oxygen bubble size and the speed of the impeller, which is complicated and not easy to control.
Other systems and methods for oxygenating for wastewater treatments have developed, for instance, U.S. Pat. Nos. 6,135,430, 6,145,815, 9,656,218, 9,486,750, 7,455,776 and 7,497,949. However, they lack of oxygen recovery.
Undissolved gas normally escapes the liquid or wastewater which is a waste. A system for treating an aeration pond with oxygen recycling is disclosed in U.S. Pat. No. 6,270,681. The system treats water with substantially pure oxygen, in which the pure oxygen originates from a pressurized oxygen supply and is conducted through a conduit to the bottom of the pond and a hood disposed to float above the surface of the pond collects Undissolved oxygen that flows back to the conduit of the pure oxygen, where the undissolved oxygen mixes with fresh pure oxygen from the pressurized source, and is recycled through the pond.
There is, therefore, a need to develop novel floating surface aerator devices for use in biotreatment of the pollutants in wastewater treatment plants, which may provide high efficiency of wastewater treatment with oxygen recycling.